So my system is sufficient to work on video materials?
I sometimes think that had i not been scanty of extra 200 i would've gotten an intel processor instead.
It seems it would make more difference..... Or not.

The source format of your video is extremely important. I mostly work in HDV which edits very smooth even on older computers. I also use AVCHD which now edits smoothly on modern computers but at one time had a lot of lag. Formats from still image DSLR cameras like the Canon 5D/7D are now the challenging formats that still have lag.

[Pavel Kuk]"Perhaps some settings in prefereces need tobe changed? "

Your project settings should match your video format. If they don't, Vegas Pro will try and convert the video on-the-fly to match the project making editing very jerky at times. So make sure that you match your project to your media.

[Pavel Kuk]"So my system is sufficient to work on video materials? "

Yes. While an Intel Core i7 would have been better, your system should be fine. Your video card is one of the BEST cards to use with Vegas Pro. I have the ATI Radeon HD 5870 and it is faster than my $800 NVIDIA Quadro 4000. Using the Sony GPU Test (i.e., the "Red Car" test) my Quadro 4000 slowed down at every transition but my Radeon HD 5870 played them straight through at full frame rate.

So depending on how demanding your video format is to decode, and if it matches your project, you may get a very different editing experience. Other NLE's like Final Cut Pro X pre-render video in the background to keep editing as smooth as possible. Vegas Pro does not do this automatically (you can use Shift+M to manually force it to do this) so the user experience isn't as smooth.